Don't like it, this sort of thing should:

1. Be discussed first, probably via an RFC
2. Not be done before we see what sort of fallout the exact same thing will have on the Linux project.

There is alread a .github/CONTRIBUTING.md file:

https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md

It could definitely use improvements though!

Be discussed first, probably via an RFC

👍. Although this means a lot more work for the author I think that altering contribution guidelines is a fundemental change where an RFC is appropriate.

I think that the contributing guidelines you're adding/modifying here should mention as well that in case of really fundamental changes an RFC should be written first :)

As pointed out by @matthewbauer the file already exists. If you can think of improvements, please go ahead and open a PR, issue, or RFC depending on the scope of the changes.